Spread-spectrum pilot signals in a communications transmitter

ABSTRACT

Interference-reducing circuits include a feed-forward circuit for subtracting all or part of a desired transmitter-signal component from a signal coupled from the power amplifier&#39;s output path. The error signal from this feed-forward circuit contains a replica of distortion in the power amplifier output. A cancellation loop adjusts the phase and/or amplitude of this error signal and combines this adjusted error signal with an interference-carrying signal, removing some of the undesired distortion. A spread-spectrum pilot signal is used in one or both loops, to provide a reference signal that can be monitored by an adjustment circuit, which measures the magnitude and/or phase of a spread-spectrum signal that is injected into the interference-carrying signal and, based on that measurement, adjusts the amplitude, phase, and/or delay of the cancellation signal that is added to the interference-carrying signal. This yields a reduced-interference signal in which undesired distortion is reduced.

This application is a continuation of prior application Ser. No.13/046,107, filed 11 Mar. 2011, the disclosure of which is incorporatedherein in its entirety.

TECHNICAL FIELD

The present invention relates generally to radio communication devicesand relates in particular to methods and apparatus for reducingundesired emissions from a power amplifier in a radio transmitter.

BACKGROUND

In radio communications systems, a transmitter transmits signals thatare picked up and processed by a receiver at some distance from thetransmitter. A few systems include only one transmitter and one or morecorresponding receivers, and thus support only one-way communications. Abroadcast television system can be viewed in this way, where atransmitter corresponding to a single television station sends one-waysignals to multiple television receivers. Similarly, a few communicationsystems include only a single receiver that monitors the signalstransmitted from several transmitters. Again, these systems supportcommunications in only a single direction. However, there are manycommunication systems that require bi-directional communication—thesesystems require both a transmitter and receiver at each system endpoint,as shown in transceivers 110 and 150, in FIG. 1. Often, the transmitterand receiver for a given endpoint, such as transmitters 115 and 165 andreceivers 125 and 155 in FIG. 1, are housed in the same device, and mayshare one or more antennas.

If both transmitters in a communications link are transmittingsimultaneously, there exists the potential that either receiver isunable to receive signals, due to interference from its correspondingtransmitter or from other transmitters. A transmitter is designed totransmit a large enough power signal to overcome the loss inherent intransmitting over a distance so that the signal can still be received.Conversely, a receiver is designed to be sensitive to extremely smallsignals to ensure that a transmitter can minimize the amount of powertransmitted over a specified distance. Because of the receiver'ssensitivity, it is possible for an unexpectedly large signal tointerfere with the receiver operation, or even to physically damagesensitive radio components. Since the transmitter in a radio system isby necessity a high-power signal, it may easily interfere with aco-located receiver in the absence of careful system design.

Of course, strong signals that are not part of the normal operation ofthe communication system can also interfere with the receiver. Possibleinterferers include television transmitters, radar systems, electricalnoise from industrial facilities, or other communication systems. Theremay be more than one interferer present. Thus, in addition tocoordinating transmissions and reception within a given system, awell-designed system must also be designed to accommodate interferencearising outside the system. Further, the system should be designed tominimize potential interference to other radio systems.

In radio transceivers, a power amplifier is used to amplify the transmitsignal to an appropriate level for transmission across the air (orthrough a cable or waveguide). The power amplifier, like any otheractive device in a signal processing chain, adds noise and distortion tothe signal. However, because the power amplifier signals are generallyquite large, the noise and distortion introduced by the power amplifiercan be particularly pronounced, especially when the power amplifier isdesigned to minimize power consumption. Power amplifier linearizationtechniques may be used to reduce the distortion introduced by the poweramplifier, but this generally does not impact the lower-level broadbandnoise emitted by the amplifier. Furthermore, specifications for a givenradio system and/or government regulations may impose stricterrequirements on emissions from the power amplifier, particularly withrespect to undesired out-of-band emissions, than are readily achievablethrough linearization techniques.

The noise and distortion emissions from the power amplifier can beviewed as an “error signal” added by the power amplifier to the desiredtransmit signal. Generally it is necessary to remove a large portion ofthis error signal before the signal is transmitted into free space viaan antenna. In a conventional radio system, a transmit filter, which maybe part of a radio duplexer circuit, serves the function of rejectingthese error signals.

One technique that is sometimes used to “linearize” a power amplifier'sresponse is called “pre-distortion.” With this technique, a poweramplifier input signal is enhanced with another signal that effectivelyanticipates the distortion produced by the PA. This pre-shaping of thepower amplifier's input signal to anticipate the distortion by the poweramplifier can significantly reduce the effective distortion of thedesired transmit signal. Both analog and digital pre-distortiontechniques are possible.

One method for coordinating the transmission and reception of signals toreduce the impact of undesired emissions is to have the transmitters andreceivers for a given system all operate on the same frequency, and thencoordinate which radio system is receiving while the other istransmitting, and vice versa. This approach is called time-divisionduplex (TDD) communication. A portion of a radio transceiver 200suitable for use in a TDD system is shown in FIG. 2. The transmitterside of radio transceiver 200 includes a power amplifier 210 and acirculator 220, while the receiver side includes a low-noise amplifier230. Both sides are coupled to an antenna path through switch 240; theantenna path comprises filter 250, antenna cable 260, and antenna 270.

As seen in the radio transceiver 200, in a TDD system both the receiverand transmitter at a given endpoint can use the same filter pass-band toreject interfering signals picked up by the antenna 270 from outsidesources, in other frequency ranges. However, other methods must be foundto ensure that radio systems within the communication system do notinterfere with one another. In a TDD system, the timing of radiotransmission and reception is carefully managed so that a transmitter istransmitting when a receiver at the other radio system is receiving andvice versa.

Another approach to coordination is called frequency-division duplex(FDD) communication, in which transmissions are separated by frequency.Thus, the transmitter of one radio system is on one frequency (e.g.,f1), and the corresponding receiver (or receivers) at the other end ofthe communication link is tuned to the same frequency. At the same time,the transmitter and corresponding receiver (or receivers) of the otherradio system is tuned to another frequency (e.g., f2). Because thetransmitter and receiver pairs are on different frequencies, filters cannow be used to ensure the transceivers do not interfere with each other,as well as to reject other interfering signals. This is shown in FIG. 3,which illustrates a radio transceiver 300 capable of full,frequency-division duplexed communication. Outgoing transmissions frompower amplifier 210 are separated in frequency from incomingtransmissions for low-noise amplifier 230, and are separately filteredby transmit filter 315, which reduces noise in the receiver band, andreceiver filter 320, which rejects emissions in the transmitter band.Together, these filters form duplexer 320 (sometimes called a diplexer).

FIG. 4 illustrates an alternative configuration for a frequency-divisionduplexed radio transceiver 400, in which the receiver and transmittereach has its own antenna 270. Very limited isolation between thetransmitter and receiver is provided by the physical separation of theantennas 270. The transmit filter 410 and receive filter 420 provideadditional isolation, as each can be tuned to reject the frequencypassed by the other. FIG. 5 illustrates yet another configuration for afrequency-division duplexing transceiver 500, in which circulator 220serves as the duplexing element. Circulator 220 provides some isolationbetween the transmitter and receiver components; additional isolation isprovided by the transmit filter 510, which is configured to reject noisein the receiver band, and receiver filter 520, which is configured toreject transmitter band emissions.

In an FDD system, electronic cancellation circuits may replace some orall of the transmit and receive filter functionality. An example of anelectronic duplex filter is described in U.S. Pat. No. 7,702,295, issued20 Apr. 2010 to Nicholls et al., the entire contents of which areincorporated by reference herein to provide background for thedisclosure that follows. In a TDD system, electronic cancellationcircuits may assist with isolating the transmit signals from thesensitive receiver circuitry.

SUMMARY

Described in detail below are techniques for adapting spread-spectrumpilot signals to an electronic duplexer cancellation system. Several ofthese techniques exploit the fact that the lower power-density of aspread-spectrum pilot signal reduces the potential for the pilot itselfto interfere with received signals. This also allows the pilot to bepositioned in a bandwidth that overlaps a related receiver band,facilitating a more accurate characterization and more completecancellation of receiver band noise than can be achieved using acontinuous-wave pilot that is offset from the receiver band.

Interference-reducing circuits according to several embodiments of thepresent invention include two cancellation loops. A first loop is afeed-forward circuit for subtracting all or part of a desiredtransmitter-signal component from a signal coupled from the poweramplifier's output path. This feed-forward circuit produces an errorsignal that contains a replica of the noise and distortion in the poweramplifier output. A second cancellation loop adjusts the phase and/oramplitude of this error signal and combines this adjusted error signalwith an interference-carrying signal, to remove all or part of theundesired noise and distortion from the interference-carrying signal.

In several embodiments, a spread-spectrum pilot signal is used in one orboth of the loops, to provide a reference signal that can be monitoredby an adjustment circuit. An adjustment circuit is configured to measurethe magnitude and/or phase of a spread-spectrum signal that is injectedinto the interference-carrying signal and, based on that measurement, toadjust the amplitude, phase, and/or delay of the cancellation signalthat is added to the interference-carrying signal. When properlyadjusted, the combination of the cancellation signal with theinterference-carrying signal yields a reduced-interference signal, inwhich undesired distortion and/or noise is substantially reduced. Theuse of a spread-spectrum pilot greatly simplifies the measurementprocess as well as reducing the potential for introducing undesirednoise into the system.

In some embodiments, a spread-spectrum pilot is applied to the transmitsignal and also to a reference signal that represents the desired outputfrom the power amplifier. This approach is particularly useful wherepre-distortion techniques are used. The reference signal is a replica ofthe desired output from the power amplifier. Separately applying aspread-spectrum pilot to the reference signal and to the power amplifierpath allows the relative amplitude and phase of the power amplifier'soutput to be tracked, so that a phase-adjusted and/or amplitude-scaledversion of the reference signal can be subtracted from that output, tocreate an error signal that has the desired transmit signal cancelledout and only contains the error signal information, i.e., residualdistortion and noise.

Another spread-spectrum pilot signal can be applied to the output signalof the power amplifier and to the error signal so that phase/amplitudecontrol of the error signal can be applied, via the second loop, tocancel the error signal from an interference-carrying signal in thepower amplifier's output path or in a path coupled to the poweramplifier output (e.g., in a receive path separated from the poweramplifier by a duplexer). Thus, as described in further detail below, apilot signal can be introduced to the main path and to the error signalpath of each of one or more cancellation loops, to facilitatephase/amplitude control of the error signal for optimalcorrection/cancellation.

Details of various circuits and methods for reducing undesired emissionsfrom a power amplifier according to the above-summarized techniques arealso disclosed. Of course, those skilled in the art will appreciate thatthe present invention is not limited to the above features, advantages,contexts or examples, and will recognize additional features andadvantages upon reading the following detailed description and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a bi-directional radio communication system.

FIG. 2 is a block diagram of a time-division-duplexing radiotransceiver.

FIG. 3 is a block diagram of a frequency-division-duplexing (FDD) radiotransceiver.

FIG. 4 is a block diagram of another FDD radio transceiver.

FIG. 5 is a block diagram of still another FDD radio transceiver.

FIG. 6 is a schematic diagram illustrating an example circuit forreducing undesired emissions from a power amplifier in a radiotransceiver.

FIG. 7 is a schematic diagram illustrating an example circuit formeasuring the presence of a spread-spectrum pilot signal in an errorsignal.

FIGS. 8-12 are each schematic diagrams illustrating example circuits forreducing undesired emissions from a power amplifier in a radiotransceiver.

FIG. 13 is a process flow diagram illustrating an example method forreducing undesired emissions from a power amplifier in a radiotransceiver.

FIG. 14 is a process flow diagram illustrating details of a process forforming an error signal.

FIG. 15 is a process flow diagram illustrating further details of anexample method for reducing undesired emissions from a power amplifierin a radio transceiver

DETAILED DESCRIPTION

In the discussion that follows, several embodiments of the presentinvention are described herein with respect to radio transceiversadapted for over-the-air transmission, such as may be employed incellular telephones or cellular base stations. However, the invention isnot so limited, and the inventive concepts disclosed and claimed hereinmay be advantageously applied in other contexts as well, including, forexample, communications transceivers coupled to waveguides, coaxialcable, and the like, operating at radio frequencies, microwavefrequencies, or even at higher frequencies. Those skilled in the artwill appreciate that the detailed design of a radio transceiver may varyaccording to its intended use, relevant standards, and/or according tocost-performance tradeoffs specific to a given manufacturer, but thatthe basics of these detailed designs are well known. Accordingly, thosedetails that are unnecessary to a full understanding of the presentinvention are omitted from the present discussion.

Furthermore, those skilled in the art will appreciate that the use ofthe term “exemplary” is used herein to mean “illustrative,” or “servingas an example,” and is not intended to imply that a particularembodiment is preferred over another or that a particular feature isessential to the present invention. Likewise, the terms “first” and“second,” and similar terms, are used simply to distinguish oneparticular instance of an item or feature from another, and do notindicate a particular order or arrangement, unless the context clearlyindicates otherwise. Further, the term “step,” as used herein, is meantto be synonymous with “operation” or “action.” The description herein ofa sequence of steps does not imply that these operations must be carriedout in a particular order, or even that these operations are carried outin any order at all, unless the context or the details of the describedoperation clearly indicates otherwise.

As discussed above, electronic cancellation circuits may be used toreplace or augment radio-frequency (RF) filter functionality incommunications transceivers. In some of these electronic cancellationcircuits, pilot signals are used to assist with automated tuning of thecancellation circuits. This is shown in the Nicholls patent referencedearlier, which describes the use of continuous-wave (CW) pilot signalsto “tune” the electronic filter functionality to the correct frequency.These pilot signals provide a clean reference that the electroniccancellation circuits can more easily detect and cancel.

One problem with CW pilot signals is that they cannot be deployeddirectly in the bandwidth of a desired signal, because they would thencause interference to the desired signals that would be difficult toremove without distorting the desired signals. However, deploymentoff-frequency means that phase and amplitude information derived fromthe CW pilot is only an approximation to the on-frequency behavior ofthe circuit. Of course, it is this on-frequency behavior which generallyyields the most desirable information. Another problem with CW pilotsignals is that they require very complete cancellation, to avoid thepossibility that they interfere with normal operations of the circuit,such as automatic-gain control operations.

It is also possible, although more difficult, to directly detect andcancel the undesired emissions in the modulated transmit and receivesignals, i.e., without using a pilot signal. However, direct detectionfrom the modulated signal can be noisy, and it can be quite difficult toacquire a good quality reference signal. Using lots of averaging toreduce the noise can take a long time, which would slow down theresponse of a system.

Outside the context of electronic filters, spread-spectrum pilot signalshave been applied to feed-forward linearization of power amplifiers.This has been demonstrated, for example, in U.S. Pat. No. 5,386,198,issued 31 Jan. 1995 to Ripstrand et al., the entire contents of whichare incorporated herein by reference to provide background for thedisclosure that follows.

The techniques described herein adapt spread-spectrum pilot signals toan electronic duplexer cancellation system. In particular, several ofthese techniques exploit the fact that a spread-spectrum pilot's energycan be distributed over a relatively large frequency band (e.g., severaltimes larger than the information bandwidth of the transmitted signals),reducing the potential for the pilot itself to interfere with receivedsignals. The low power-density of the spread-spectrum pilot signalallows it to be positioned in a bandwidth that overlaps a receiver bandof the communications transceiver, allowing a more accuratecharacterization and more complete cancellation of receiver band noisethan can be achieved with a CW pilot that is offset from the receiverband.

More particularly, an interference-reducing circuit according to severalembodiments of the present invention includes two cancellation loops. Afirst loop is a feed-forward circuit for subtracting all or part of adesired transmitter-signal component from a signal coupled from thepower amplifier's output path. This feed-forward circuit produces anerror signal that contains a replica of the noise and distortion in thepower amplifier output. A second cancellation loop adjusts the phaseand/or amplitude of this error signal and combines this adjusted errorsignal with an interference-carrying signal, to remove all or part ofthe undesired noise and distortion from the interference-carryingsignal.

The interference-carrying signal can be at any of several places in theradio transceiver. For instance, the interference-reducing circuit maybe configured to eliminate undesired power amplifier emissions as closeto the source as possible—in this case, the cancellation of theemissions with the adjusted error signal can take place in the poweramplifier's output path, i.e., before any duplexing element.

In other embodiments, the circuit may be instead configured primarily toeliminate undesired emissions that have “leaked” into the receiver inputpath—in this case, the cancellation of the undesired transmitteremissions may be positioned in the receiver path, e.g., between anantenna duplexer element and a low-noise amplifier in the receive path.It is also possible to position the cancellation on the antenna side ofthe duplexer, although this approach may be undesirable due to the extralosses in the antenna path introduced by the coupling of thecancellation signal to the antenna path—with this approach, any lossesintroduced by the coupling affect both the transmit path and the receivepath. In still another approach, cancellation may be introduced intoboth the power amplifier path and the receiver input path—this approachrequires an additional cancellation loop, as described in further detailbelow.

In several embodiments of these circuits, a spread-spectrum pilot signalis used in one or both of the loops, to provide a reference signal thatcan be monitored by an adjustment circuit. For instance, an adjustmentcircuit can be configured to measure the magnitude and/or phase of aspread-spectrum signal that is injected into the interference-carryingsignal and, based on that measurement, to adjust the amplitude, phase,and/or delay of the cancellation signal that is added to theinterference-carrying signal. When one or more of these parameters ofthe cancellation signal is properly adjusted, the combination of thecancellation signal with the interference-carrying signal yields areduced-interference signal, in which undesired distortion and/or noiseis substantially reduced. The use of a spread-spectrum pilot greatlysimplifies the characterization of the amplitude, phase, and/or delay ofthe cancellation, and also reduces the potential for introducingundesired noise into the system.

In some embodiments, then, a spread-spectrum pilot is applied to boththe transmit signal and also to a reference signal that represents thedesired output from the power amplifier. This approach is particularlyuseful where pre-distortion techniques are used. In this case, the shapeof the signal input to the power amplifier is appreciably different(intentionally) from the shape of the power amplifier's output. To bestremove residual distortion and noise that is not accounted for by thepre-distortion, it is necessary to first remove the desired signalcomponents from the power amplifier output. Thus, the reference signalis a replica of the desired output from the power amplifier. Separatelyapplying a spread-spectrum pilot to the reference signal and to thepower amplifier path allows the relative amplitude and phase of thepower amplifier's output to be tracked. In this manner, a phase-adjustedand/or amplitude-scaled version of the reference signal can besubtracted from that output, to create an error signal that has thedesired transmit signal cancelled out and only contains the error signalinformation, i.e., residual distortion and noise.

Another spread-spectrum pilot signal can be applied to the output signalof the power amplifier and to the error signal so that phase/amplitudecontrol of the error signal can be applied, via the second loop, tocancel the error signal from an interference-carrying signal in thepower amplifier's output path or in a path coupled to the poweramplifier output (e.g., in a receive path separated from the poweramplifier by a duplexer). Thus, as described in further detail below, apilot signal can be introduced to the main path and the error signalpath of each of one or more cancellation loops, to facilitatephase/amplitude control of the error signal for optimalcorrection/cancellation.

In some cases, multiple pilot signals may be simultaneously needed in agiven signal path, to capture separate information, such as in separatefrequency regions. In this case, different spreading codes may be usedto uniquely identify and separate the pilot signals, particularly whenthe pilot signals occupy overlapping frequency bands.

It will be recognized by those skilled in the art that there are manydifferent places the pilot signal could be injected into the circuit,and likewise many points where the pilot signals could be detected. Thedecision of where to put injection and detection points implies manytradeoffs that are part of the overall radio system design. Several ofthese possible approaches are described in detail herein and illustratedin the attached figures. However, the illustrated embodiments should notbe understood to limit the applicability of the techniques disclosedherein, as the same techniques may be applied to variations of thespecifically illustrated circuits.

FIG. 6 is a simplified schematic diagram of an example cancellationcircuit 600, illustrating a signal flow applicable to severalembodiments of the present invention. In this embodiment, a firstspread-spectrum pilot signal, SSP1, is combined with a transmit signal,TX, using coupling circuit 610. In practice, combiner 610 could be adirectional coupler, a summing amplifier, or any other device or circuitfor coupling radio-frequency signals, including components thateffectively add two signals together as well as components that“subtract” one signal from another.

The spread-spectrum pilot signal SSP1, in several embodiments, comprisesa continuous-wave signal that has been spread, or “chipped,” by apseudo-random digital sequence. This spreading technique, which iscommonly known as direct-sequence spread spectrum, creates a signal inwhich the CW pilot's energy is spread out over a bandwidth comparable tothe chipping rate. With an appropriate spreading signal, the resultingbandwidth of the spread-spectrum pilot signal can be several Megahertz,or even tens of Megahertz.

In this embodiment, the purpose of SSP1 is to track the phase andamplitude differences between the power amplifier and cancellationsignal paths in the first (leftmost) cancellation loop of FIG. 6. Thisallows the removal of a desired transmitter signal component, to producean error signal that contains primarily unwanted distortion and noise.Because this first loop is designed to characterize in-band performanceof the transmitter path, the spread-spectrum pilot signal is mosteffective if positioned in the transmitter band. Accordingly, in severalembodiments the bandwidth of SSP1 is configured so that it at leastpartly overlaps the bandwidth of the desired transmitter signal. This isdone by selecting a CW pilot (i.e., the pilot signal before spreading)at a frequency in or very near the transmitter frequency band.

A second spread-spectrum signal SSP2 is also combined with the TXsignal, using combiner 610. (Those skilled in the art will appreciatethat combiner 610 may comprise a four-port device, as suggested in FIG.6, or two separate three-port devices.) As discussed in further detailbelow, this second spread-spectrum pilot signal may be used tocharacterize unwanted noise and distortion in a receiver frequency band.Thus, in several embodiments this spread-spectrum pilot signal SSP2 isdesigned to at least partly overlap a receiver frequency band. To allowfor easy distinguishing between the two pilot signals, the spreadingsequences used to create SSP1 and SSP2 may be selected from a group ofspreading sequences that are orthogonal to one another, in someembodiments.

In the circuit configuration illustrated in FIG. 6, SSP1 is alsocombined with a reference signal REF, using combiner 620. As brieflydiscussed earlier, this reference signal REF is a replica of a desired,or expected, output of the power amplifier 630. This signal may lookquite different than the TX signal, which may be pre-distorted toaccount in advance for the non-linear distortion introduced by poweramplifier 630.

The result of combining REF and SSP1 is labeled signal S3, in FIG. 6.The summation of TX, SSP1 and SSP2 is labeled signal S1. As noted above,the signal TX itself is related to signal REF and may or may not be apre-distorted version of REF.

Once signal S1 is amplified by the power amplifier 630, then theamplified signal S2 includes a distorted version of TX, plus additionalnoise and distortion introduced by the power amplifier 630. Thus, signalS2 includes a desired transmit signal component that resembles REF, aswell as undesired distortion and noise. Signal S2 also includesamplified versions of SSP1 and SSP2. In general, because thespread-spectrum pilot signals SSP1 and SSP2 have peak amplitudes wellbelow those of TX, the power amplifier output does not includesignificant distortion products of SSP1 and SSP2.

Circuit 640 controls the phase, amplitude, and or delay of signal S3,and may be configured to allow one or more of these parameters to beadjusted. For instance, circuit 640 may include a variable-gainamplifier, a variable-phase circuit, or a variable-delay circuit, or anycombination of these. Likewise, circuit 650 similarly controls thephase, amplitude, and/or delay of a signal sample coupled from the poweramplifier's output path at coupler 655, which may be a directionalcoupler, for example. If the output of circuit 640 is controlled so thatthe phase of the REF signal is opposite to the corresponding poweramplifier output component at the output of 650, and so that theamplitudes of these two signals are the same, then the combining ofthese two signals in combiner 660 results in an error signal S4 thatcontains only the noise and distortion from PA, since REF is cancelledout. Of course, those skilled in the art will appreciate that combiner660 can be a subtracting combiner, in some cases, in which case theoutputs from circuits 640 and 650 should be controlled to be in-phase,rather than out-of-phase. Those skilled in the art will furtherappreciate that any of the phase, amplitude, or delay can be controlledin just one of circuits 640 and 650, in various embodiments, or in both,in other embodiments. In some embodiments, only one or two of theseparameters is dynamically adjusted, while the remaining parameters arefixed. In others, all three parameters are dynamically adjustable tomaximize the cancellation of the desired transmitter signal componentsfrom signal S4.

Only a small portion of signal S2 is expected to flow through circuit650 from the output signal tap at coupler 655. The largest portion ofsignal S2 should travel through the remainder of the radio system's maintransmitter path, which is represented in FIG. 6 as cloud 665. Cloud 665may include a duplexer or other transmitter path components, elements ofan antenna path, and/or components in a receiver signal path, in variousembodiments. In any case, the signal at the output of cloud 665comprises a replica of signal S2, with phase, amplitude, and delaychanges introduced by coupler 655 and cloud 665. In some cases, such aswhere combiner 680 is positioned in a receiver path, the desiredtransmitter signal may be substantially attenuated, e.g., by a receiveband filter. However, even in these cases the signal emerging from cloud665 is nevertheless likely to carry a substantial portion of theunwanted noise and distortion introduced by power amplifier 630, such asreceiver band noise and distortion that cannot be addressed by areceiver band filter.

As noted above, however, the error signal S4 output by combiner 660consists primarily of the unwanted noise and distortion from poweramplifier 630, since the desired signal has been canceled. Accordingly,if the phase and amplitude of this error signal are properly controlled,to match the phase, amplitude, and delay introduced in the poweramplifier's output path by coupler 655 and cloud 665, then the adjustederror signal can be combined with the output of cloud 665, usingcombiner 680, to produce an interference-reduced signal S6 in which theunwanted distortion and noise is reduced. In the circuit illustrated inFIG. 6, this is done with circuit 670, which is configured to adjust oneor more of the phase, amplitude, or delay of signal S4, to create acancellation signal S5 that is out-of-phase with the output from cloud665 and has a similar amplitude.

It will be understood to those skilled in radio-frequency circuit designthat there are a number of methods and circuits for separating offportions of a signal, i.e., sampling a radio-frequency signal, or forcombining radio-frequency signals. These circuits, which maycollectively be regarded as coupling circuits, include, but are notlimited to directional couplers, hybrid couplers, power combiners, andpower splitters. Combiners 610, 620, 660, and 680 may each comprise oneor more of these circuits, as may couplers 655, 675, and 685.

The phase, amplitude, and/or delay characteristics of the first loop arecontrolled by SSP1 measurement circuit 655, which is configured todynamically adjust circuit 640, circuit 650, or both, based on detectingthe presence of SSP1 in the error signal S4 output from combiner 660.Likewise, the phase, amplitude and/or delay characteristics of thesecond loop are controlled by SSP2 measurement circuit 690, which isconfigured to dynamically adjust circuit 670, based on detecting thepresence of SSP2 in the interference-reduced signal S6 output fromcombiner 680.

The basic function of these circuits is the same: to detect the presenceof the respective spread-spectrum pilot signal in the error signal, andto provide adjustments to phase/amplitude/delay control circuits tominimize the presence of the pilot signal in the error signal. FIG. 7illustrates one example of how such a spread-spectrum pilot measurementcircuit might be configured. First, an error signal input, correspondingto error signal S4 or to interference-reduced signal S6, in FIG. 6, is“de-spread” by multiplying it, at multiplier 710, with the samespreading sequence used to create SSP1 or SSP2, as appropriate. Thisde-spreading operation has two effects. First, the distortion and noisecomponents of the error signal are spread over a bandwidth comparable tothe spreading sequence frequency. Second, any residual component of SSP1or SSP2 in the error signal is de-spread, to form a CW tone thatreflects the difference in magnitude and phase between the two signalsthat were combined to form the error signal.

This de-spread error signal is then multiplied by in-phase andquadrature replicas of the CW tone used to originally create SSP1 orSSP2, at multipliers 720 and 730. The in-phase result, labeled “I” inFIG. 7, is filtered at low-pass filter 740 and integrated at integrator750, to produce an amplitude control signal. Similarly, thequadrature-phase result, labeled “Q” in FIG. 7, is filtered at low-passfilter 760 and integrated at integrator 770, to produce a phase controlsignal. The amplitude control signal and the phase control signal can beapplied to variable-gain and variable-phase circuits, respectively, toadjust the amplitude and phase of the cancellation loop.

The function of the circuit illustrated in FIG. 7 is to perform anenergy measurement of the spread-spectrum pilot signal, in the cartesiancoordinate plane. The cartesian coordinate plane is defined by a I/Osplit reference signal applied to multipliers 720 and 730. This circuitcan also be regarded as an I/O demodulator or a matched filtercorrelator. For proper operation, it is assumed that the time alignment(delay) between the CW tones at multipliers 720 and 730 and the errorsignal are set properly, using delay elements that are not shown.

Given that assumption, the CW tone input to multiplier 720 takes theform R=cos(ωt), while the de-spread error signal has the formD=cos(ωt)−A cos(ωt+φ). A represents the gain of the cancellation loop,while φ is the phase of the loop. ω is the angular frequency of the CWtone. Then:

$\begin{matrix}{I = {{\cos^{2}( {\omega\; t} )} - {\frac{A}{2}{\cos( {{2\;\omega\; t} + \varphi} )}} - {\frac{A}{2}\cos\;\varphi}}} \\{= {\frac{1}{2} - {\frac{A}{2}\cos\;\varphi}}} \\{{= {\frac{1}{2}\lbrack {1 - {A\;\cos\;\varphi}} \rbrack}},}\end{matrix}$ and: $\begin{matrix}{Q = {{{\sin( {\omega\; t} )}{\cos( {\omega\; t} )}} - {A\;{\sin( {\omega\; t} )}{\cos( {{\omega\; t} + \varphi} )}}}} \\{= {{- \frac{A}{2}}\sin\;{\varphi.}}}\end{matrix}$

For small angles φ, signals I and Q can be integrated by integrators 770and 790 to yield amplitude and phase control signals, respectively. Theeasy integration is possible due to the fact that when the system issettled the angle φ is equal to zero, while the amplitude A is equalto 1. Of course, the dynamics of this control system are affected by thephase and amplitude responses of the corresponding loop, as a functionof frequency and power. The loop filters and integrators in FIG. 7 canbe readily designed to accommodate these dynamics, using techniques wellknown to those conversant in control theory. Of course, other techniquesfor measuring the presence of the spread-spectrum pilot signal in theerror signal, including digital techniques in which the error signal isdigitized and analyzed to detect the amplitude and/or phase of anyresidual portion of the spread-spectrum pilot signal. A digitalimplementation provides a great degree of flexibility in defining andimplementing the control problem, which can be viewed as an optimization(e.g., minimization) of the presence of the pilot signal in the errorsignal. Hence, well-known optimization algorithms such as gradientdescent or simplex method may be applied. Techniques for adaptivefiltering, such as Kalman filters or similar techniques, might also beapplied.

As was noted earlier, it is possible to inject the spread-spectrum pilotsignal(s) at several places in any given cancellation circuitconfiguration. For example, FIG. 8 illustrates a cancellation circuithaving the same general configuration as that of FIG. 6. However, in thecircuit of FIG. 8, the spread-spectrum pilot SSP2 is introduced afterthe sampling of the power amplifier's output signal at coupler 655,using an additional coupler 810. Because SSP2 is not present in thesampled signal path in this case, it must also be injected into theerror signal, via combining circuit 660. In another embodiment, shown inFIG. 9, spread spectrum pilot SSP2 is injected into the poweramplifier's output path via a coupler 910 positioned at the output ofpower amplifier 630, but before the output signal tap at coupler 655.

The cancellation of unwanted noise and distortion can also be positionedat several different places in a transceiver circuit. For instance, inFIG. 10, the cancellation circuit is configured to cancel emissionsbefore the antenna of a radio system. Thus, the combining of the errorsignal with the power amplifier's output signal is performed at combiner1010, which is positioned between the power amplifier 630 and a duplexer1020. This configuration also reduces transmitter emissions leakingthrough duplexer 1020 to receiver low-noise amplifier 1030. Note thatFIG. 10 and subsequent figures omit for clarity several featurespictured in earlier figures, such as the spread-spectrum pilotmeasurement circuits; the location and configuration of these missingdetails will be apparent given the more detailed diagrams in FIGS. 6, 8,and 9.

In another approach, shown in FIG. 11, the cancellation loop is insteadused to cancel emissions after the antenna duplexer 1020, via combiner1110, but before the low-noise amplifier 1030 on the receiver side ofthe transceiver system. In other embodiments, the cancellation couldinstead be positioned after the low-noise amplifier 1030.

In still another embodiment, shown in FIG. 12, two interferencecancellation loops are used, in addition to the first loop used togenerate the error signal. The first cancellation loop is used to cancelemissions before the antenna on the transmit side of the radio system,in the same manner illustrated in FIG. 10. A second cancellation loop isalso used, to further cancel emissions after the antenna on the receiverside of the radio system. In the pictured circuit, a thirdspread-spectrum pilot SSP3 is used, and injected into the secondinterference-cancelling loop via combiner 1210. The secondinterference-cancelling loop further includes phase/amplitude circuits1220 and 1240, the latter of which may be used in some embodiments toadjust the phase and/or amplitude of a second error signal provided viacombiner 1230. The second cancellation is performed at combiner 1250,just before low-noise amplifier 1250. Not shown is a measurement circuitlike those in FIGS. 6, 8, and 9; the omitted measurement circuit shouldbe configured to measure the presence of SSP3 in the output of combiner1250 (either before or after low-noise amplifier 1030) and to adjust thephase, amplitude, or delay of the loop via circuits 1220 and/or 1240.

The pilot SSP3 could be the same source as SSP2, since the firstinstance of SSP2 would be cancelled in the main path by the correctionprocess. Alternatively SSP3 could be a new spread-spectrum pilot using adifferent spreading code, in which case SSP3 could be detectedindependently using spread spectrum techniques and SSP2 would notinterfere with SSP3 should any residual of SSP2 not be cancelled outthrough some error in the correction loop. Note that it is also possiblefor SSP2 and SSP3 to be positioned to cover different frequency ranges,to optimize cancellation of unwanted emissions at different points ofthe frequency spectrum.

The circuits of FIGS. 6-12 comprise example implementations of severaltechniques for reducing undesired emissions from a power amplifier in acommunications transceiver. These techniques can alternatively beembodied as methods or processes. For instance, an example method forreducing undesired emissions from a power amplifier in a communicationstransceiver is illustrated in FIG. 13. The first “step” is shown atblock 1310, and includes subtracting all or part of a desiredtransmitter-signal component from a sampled signal coupled from thepower amplifier's output signal path, to create an error signal. (Notethat while the current discussion characterizes various operations as“steps,” this is not meant to imply that these operations are carriedout in a particular order. In fact, it will be apparent to those skilledin the art that many of these operations are in fact carried outsimultaneously.) This step corresponds to the left-most loop in thecircuits illustrated in FIGS. 6 and 8-12.

Although a spread-spectrum pilot need not be used in this first loop inevery embodiment, in some embodiments this subtraction operation followsthe process illustrated in FIG. 14, which comprises adding aspread-spectrum pilot signal to a reference signal that represents adesired output from the power amplifier, to form a comparison signal, asshown at block 1410; adding the second spread-spectrum pilot signal to amodulated transmitter signal, to form an input signal for the poweramplifier, as shown at block 1420; adding the comparison signal to thesampled signal, to form the error signal, as shown at block 1430;measuring the spread-spectrum pilot signal in the error signal, as shownat block 1440; and adjusting at least one parameter of the comparisonsignal or the input signal to the power amplifier, based on thismeasurement, to reduce the presence of the reference signal in the errorsignal, as shown at block 1440. In several embodiments, thespread-spectrum pilot signal occupies a bandwidth at least partlyoverlapping the bandwidth occupied by the reference signal, tofacilitate accurate characterization of the loop's response at thetransmitter frequency.

Referring back to FIG. 13, the next step in the cancellation processaccording to the pictured method is to inject a spread-spectrum pilotsignal into the power amplifier's output signal path and into the errorsignal, as shown at block 1320. This spread-spectrum pilot signaloccupies a bandwidth at least partially overlapping a receiver band ofthe communications transceiver, to facilitate an accuratecharacterization of the cancellation loop's response at the receivefrequency. In some embodiments, as discussed earlier, thespread-spectrum signal comprises a pilot base signal (e.g., acontinuous-wave tone) multiplied by a spreading sequence. However, otherstructures for the spread-spectrum pilot signal are possible, includingthose in which the pilot base signal is a modulated signal, rather thana CW tone.

The spread-spectrum pilot signal can be injected into the poweramplifier's output signal path in several ways. For instance, it can becoupled to an input signal to the power amplifier, or added to the poweramplifier's output signal path at a point after the sampled signal iscoupled from the power amplifier's output signal path. In otherembodiments, the spread-spectrum pilot signal is added to the poweramplifier's output signal path at a point after the power amplifier butbefore the sampled signal is coupled from the power amplifier's outputsignal path.

Next, as shown at block 1330, the error signal produced by the precedingoperations is added to an interference-carrying signal to form areduced-interference signal, wherein the interference-carrying signal isin the power amplifier's output signal path or is coupled to the poweramplifier's output signal path. For instance, in some embodiments theerror signal is added to a receive-path signal coupled to the poweramplifier's output signal path by a duplexer circuit, to form thereduced-interference signal. In others, the error signal is added to thepower amplifier's output signal path, to form the reduced-interferencesignal, at a point before a duplexer circuit.

The method further comprises measuring the first spread-spectrum signalin the reduced-interference signal, as shown at block 1340. In someembodiments, this comprises de-spreading the reduced-interferencesignal, using the spreading sequence used to create the spread-spectrumpilot signal, and detecting a peak magnitude in the de-spread signal.Based on the measurement, at least one parameter of the first errorsignal or the power amplifier output signal is adjusted, as shown atblock 1350, to reduce the presence of the first error signal in thereduced-interference signal. This at least one parameter that isadjusted can include one or more of the signal phase, the signalamplitude, and the signal delay. Thus, for example, a variable gainamplifier may be adjusted, in some embodiments, while a variable-phaseelement is adjusted in another. In some embodiments, both aresimultaneously adjusted.

FIG. 15 illustrates the operations associated with the use of a secondcancellation loop, for multi-loop cancellation, as was illustrated inthe schematic diagram of FIG. 12. The operations of FIG. 15 may becarried out in conjunction with those of FIG. 13, and include, as shownat block 1510, injecting a third spread-spectrum pilot signal into thereduced-interference signal, at a point before the duplexer circuit. Asecond error signal is then formed by subtracting all or part of thedesired transmitter-signal component from a second sampled signalcoupled from the power amplifier's output signal path at a point afterthe injection of the second spread-spectrum pilot signal, as shown atblock 1520. This second error signal is added to a receive-path signalcoupled to the power amplifier's output signal path by a duplexercircuit, to form a reduced-interference receiver signal, as shown atblock 1530, and the second spread-spectrum pilot signal is measured inthe reduced-interference receiver signal, as shown at block 1540. Basedon this measuring, at least one parameter of the second error signal isadjusted, to reduce the presence of power amplifier emissions in thereduced-interference receiver signal.

It will be understood by those skilled in the art that the methods andcircuits described above for using spread-spectrum pilot signals forreducing interference are also applicable to radio systems whereseparate antennas are used for receive and transmit signals. Further, itwill be appreciated by those skilled in the art that the schematicdiagrams of FIGS. 6-11 are necessarily simplified, to clarify thepresentation of the inventive techniques disclosed herein, and do notinclude, for example, delay elements that might be necessary or detailsof the phase changes and amplitude adjustments. While such elements arenecessary for correct operation of the circuit, the precise nature ofthose elements is highly dependent on the intended frequency ofoperation, power level and other parameters relevant to the applicationof a radio system. Moreover, the techniques for designing andimplementing these features are well known to those skilled in the artof radio transceiver design.

Although the present inventive techniques are generally described abovein the context of a radio transceiver, those skilled in the art willappreciate that these techniques are more generally applicable tocommunications transceivers utilizing a power amplifier. Indeed, giventhe above variations and examples in mind, those skilled in the art willappreciate that the preceding descriptions of various embodiments ofmethods and circuits are given only for purposes of illustration andexample. One or more of the specific processes discussed above may becarried out in a cellular phone or other communications transceivercomprising one or more appropriately configured processing circuits,which may in some embodiments be embodied in one or moreapplication-specific integrated circuits (ASICs). In some embodiments,these processing circuits may comprise one or more microprocessors,microcontrollers, and/or digital signal processors programmed withappropriate software and/or firmware to carry out one or more of theoperations described above, or variants thereof. In some embodiments,these processing circuits may comprise customized hardware to carry outone or more of the functions described above. Those skilled in the artwill recognize, of course, that the present invention may be carried outin other ways than those specifically set forth herein without departingfrom essential characteristics of the invention. The present embodimentsare thus to be considered in all respects as illustrative and notrestrictive, and all changes coming within the meaning and equivalencyrange of the appended claims are intended to be embraced therein.

What is claimed is:
 1. A method for reducing undesired emissions from apower amplifier in a communications transmitter, the method comprising:using a feed-forward circuit, subtracting all or part of a desiredtransmitter-signal component from a first sampled signal coupled fromthe power amplifier's output signal path, to create a first errorsignal; injecting a first spread-spectrum pilot signal into the poweramplifier's output signal path and into the first error signal, whereinthe first spread-spectrum pilot signal occupies a first bandwidth atleast partially overlapping a receiver band; adding the first errorsignal to an interference-carrying signal to form a reduced-interferencesignal, wherein the interference-carrying signal is in the poweramplifier's output signal path or is coupled to the power amplifier'soutput signal path; measuring the first spread-spectrum pilot signal inthe reduced-interference signal; and adjusting at least one parameter ofthe first error signal or the power amplifier output signal, based onthe measuring of the first spread-spectrum pilot signal, to reduce thepresence of the first error signal in the reduced-interference signal.2. The method of claim 1, wherein the at least one parameter of thefirst error signal or the power amplifier signal comprises one or moreof a signal phase, a signal amplitude, and a signal delay.
 3. The methodof claim 1, wherein using a feed-forward circuit to subtract all or partof the desired transmitter-signal component from the first sampledsignal comprises: adding a second spread-spectrum pilot signal to areference signal that represents a desired output from the poweramplifier, to form a first comparison signal, wherein the secondspread-spectrum pilot signal occupies a second bandwidth at least partlyoverlapping the bandwidth occupied by the reference signal; adding thesecond spread-spectrum pilot signal to a modulated transmitter signal,to form an input signal for the power amplifier; adding the firstcomparison signal to the first sampled signal, to form the first errorsignal; measuring the second spread-spectrum pilot signal in the firsterror signal; and adjusting at least one parameter of the firstcomparison signal or the input signal to the power amplifier, based onthe measuring of the second spread-spectrum pilot signal, to reduce thepresence of the reference signal in the first error signal.
 4. Themethod of claim 3, wherein the at least one parameter of the firstcomparison signal or the input signal to the power amplifier comprisesone or more of a signal phase, a signal amplitude, and a signal delay.5. The method of claim 1: wherein the first spread-spectrum signalcomprises a pilot base signal multiplied by a spreading sequence;wherein measuring the first spread-spectrum signal in thereduced-interference signal comprises de-spreading thereduced-interference signal, using the spreading sequence, and detectinga peak magnitude in the de-spread signal.
 6. The method of claim 1,wherein injecting the first spread-spectrum pilot signal into the poweramplifier's output signal path comprises adding the firstspread-spectrum pilot signal to an input signal to the power amplifier.7. The method of claim 1, wherein injecting the first spread-spectrumpilot signal into the power amplifier's output signal path comprisesadding the first spread-spectrum pilot signal to the power amplifier'soutput signal path at a point after the first sampled signal is coupledfrom the power amplifier's output signal path.
 8. The method of claim 1,wherein injecting the first spread-spectrum pilot signal into the poweramplifier's output signal path comprises adding the firstspread-spectrum pilot signal to the power amplifier's output signal pathat a point before the first sampled signal is coupled from the poweramplifier's output signal path.
 9. The method of claim 1, wherein thefirst error signal is added to a receive-path signal coupled to thepower amplifier's output signal path by a duplexer circuit, to form thereduced-interference signal.
 10. The method of claim 1, wherein thefirst error signal is added to the power amplifier's output signal path,to form the reduced-interference signal, at a point before a duplexercircuit.
 11. The method of claim 10, further comprising: injecting asecond spread-spectrum pilot signal into the reduced-interferencesignal, at a point before the duplexer circuit; forming a second errorsignal by subtracting all or part of the desired transmitter-signalcomponent from a second sampled signal coupled from the poweramplifier's output signal path at a point after the injection of thesecond spread-spectrum pilot signal; adding the second error signal to areceive-path signal coupled to the power amplifier's output signal pathby a duplexer circuit, to form a reduced-interference receiver signal;measuring the second spread-spectrum pilot signal in thereduced-interference receiver signal; and adjusting at least oneparameter of the second error signal, based on the measuring of thesecond spread-spectrum pilot signal, to reduce the presence of poweramplifier emissions in the reduced-interference receiver signal.
 12. Themethod of claim 1, wherein the injecting the first spread-spectrum pilotsignal into the first error signal comprises adding the firstspread-spectrum pilot signal after the first sampled signal is coupledfrom the power amplifier's output signal path.
 13. A circuit forreducing undesired emissions from a power amplifier in a communicationstransmitter, the circuit comprising: a first feed-forward circuit loopcomprising: the power amplifier; a sampling circuit configured to couplea first sampled signal from the power amplifier's output signal path;and a first cancellation circuit configured to subtract all or part of adesired transmitter-signal component from the first sampled signal, tocreate a first error signal; a first coupling circuit configured toinject a first spread-spectrum pilot signal into the power amplifier'soutput signal path and into the first error signal, wherein the firstspread-spectrum pilot signal occupies a first bandwidth at leastpartially overlapping a receiver band; a first adder circuit configuredto add the first error signal to an interference-carrying signal eitherin the power amplifier's output signal path or coupled to the poweramplifier's output signal path, to form a reduced-interference signal; afirst pilot detection circuit configured to measure the firstspread-spectrum pilot signal in the reduced-interference signal; and afirst signal adjustment circuit configured to adjust at least oneparameter of the first error signal or the power amplifier outputsignal, based on the measuring of the first spread-spectrum pilotsignal, to reduce the presence of the first error signal in thereduced-interference signal.
 14. The circuit of claim 13, wherein thefirst signal adjustment circuit is configured to adjust one or more of asignal phase, a signal amplitude, and a signal delay.
 15. The circuit ofclaim 13, wherein the first feed-forward circuit loop comprises: asecond adder circuit configured to add a second spread-spectrum pilotsignal to a reference signal that represents a desired output from thepower amplifier, to form a first comparison signal, wherein the secondspread-spectrum pilot signal occupies a second bandwidth at least partlyoverlapping the bandwidth occupied by the reference signal; a thirdadder circuit configured to add the second spread-spectrum pilot signalto a modulated transmitter signal, to form an input signal for the poweramplifier; a fourth adder circuit configured to add the first comparisonsignal to the first sampled signal, to form the first error signal; asecond pilot detection circuit configured to measure the secondspread-spectrum pilot signal in the first error signal; and a secondsignal adjustment circuit configured to adjust at least one parameter ofthe first comparison signal or the input signal to the power amplifier,based on the measuring of the second spread-spectrum pilot signal, toreduce the presence of the reference signal in the first error signal.16. The circuit of claim 15, wherein the second signal adjustmentcircuit is configured to adjust one or more of a signal phase, a signalamplitude, and a signal delay.
 17. The circuit of claim 13: wherein thefirst spread-spectrum signal comprises a pilot base signal multiplied bya spreading sequence; wherein the first pilot detection circuitcomprises a multiplier circuit configured to de-spread thereduced-interference signal, using the spreading sequence, and apeak-detection circuit configured to detect a peak magnitude in thede-spread signal.
 18. The circuit of claim 13, wherein the firstcoupling circuit comprises a summing circuit configured to add the firstspread-spectrum pilot signal to an input signal to the power amplifier.19. The circuit of claim 13, wherein the first coupling circuitcomprises a summing circuit configured to add the first spread-spectrumpilot signal to the power amplifier's output signal path at a pointafter the first sampled signal is coupled from the power amplifier'soutput signal path.
 20. The circuit of claim 13, wherein the firstcoupling circuit comprises a summing circuit configured to add the firstspread-spectrum pilot signal to the power amplifier's output signal pathat a point before the first sampled signal is coupled from the poweramplifier's output signal path.
 21. The circuit of claim 13, furthercomprising a duplexer circuit coupling the power amplifier's output pathto an antenna signal path and to a receiver path, wherein the firstcoupling circuit comprises a summing circuit configured to add the firsterror signal to a receive-path signal in the receiver path, to form thereduced-interference signal.
 22. The circuit of claim 13, furthercomprising a duplexer circuit coupling the power amplifier's output pathto an antenna signal path and to a receiver path, wherein the firstadder circuit is configured to add the first error signal to the poweramplifier's output signal path, to form the reduced-interference signal,at a point before the duplexer circuit.
 23. The circuit of claim 22,further comprising: a second coupling circuit configured to inject asecond spread-spectrum pilot signal into the reduced-interferencesignal, at a point before the duplexer circuit; a second cancellationcircuit configured to generate a second error signal by subtracting allor part of the desired transmitter-signal component from a secondsampled signal coupled from the power amplifier's output signal path ata point after the injection of the second spread-spectrum pilot signal;a summing circuit configured to add the second error signal to areceive-path signal in the receiver path, to form a reduced-interferencereceiver signal; a second pilot detection configured to measure thesecond spread-spectrum pilot signal in the reduced-interference receiversignal; and a second signal adjustment circuit configured to adjust atleast one parameter of the second error signal, based on the measuringof the second spread-spectrum pilot signal, to reduce the presence ofpower amplifier emissions in the reduced-interference receiver signal.24. The circuit of claim 13, wherein the first coupling circuitcomprises a summing circuit configured to add the first spread-spectrumpilot signal to the first sampled signal at a point after the firstsampled signal is coupled from the power amplifier's output signal path.